What Resistor You Need For Which Colour Of LED

All LEDs are not made similar. There is a parameter named forward voltage for the diodes and LEDs. The forward voltage is the voltage drop across the diode if the voltage at the anode is more positive than the voltage at the cathode. You will be using this value to calculate the power dissipation of the diode and the voltage after the diode. Technically it is the forward voltage which defines the amount of voltage required to conduct electricity. Below the amount will cause the LED to remain non-conductive. We can measure the forward current of LED as Volt.

There is also another parameter named forward current. When forward voltage is applied to a LED, it turns into a short-circuit and allows current to flow. You must limit the amount of forward current flow through the LED. That is we say to add current limiting resistor. By placing a resistor in series with the LED, the current that is effectively limited.

From that we calculate the resistance of LED in ohm. Usually this value is closer to 13 ohm. In easy word – connect the LED with 3.3v supply voltage and the ground to multimeter to check voltage, amperage. You need resistors of various values between the LED with 3.3v supply voltage.

Typically, the forward voltage of an LED is about 1.8–3.3 volts and it varies by the color of the LED, voltage drop normally rises as the light frequency increases, so a blue LED may drop around 3.3 volts. In real, LED forward voltages :

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IR LED ~ 1.5V

Red: ~2V

Amber: ~2V

Yellow: ~2V

Green: ~2.5V

Blue: ~3.5V

White: ~3.5V

Laser diode: ~1.5V

But you need an approximate values of the resistors against LEDs common colours.

Power Supply Voltage

LED Color

LED Vf

LEDs in series

Desired Current

Resistor (calculated)

Resistor (rounded)

3 V

Red, Yellow, or Yellow-Green

1.8

1

25 mA

48 Ω

51 Ω

4.5 V

Red, Yellow, or Yellow-Green

1.8

2

25 mA

36 Ω

39 Ω

4.5 V

Blue, Green, White, or UV

3.3

1

25 mA

48 Ω

51 Ω

5 V

Blue, Green, White, or UV

3.3

1

25 mA

68 Ω

68 Ω

5 V

Red, Yellow, or Yellow-Green

1.8

1

25 mA

128 Ω

150 Ω

5 V

Red, Yellow, or Yellow-Green

1.8

2

25 mA

56 Ω

56 Ω

9 V

Red, Yellow, or Yellow-Green

1.8

4

25 mA

72 Ω

75 Ω

9 V

Blue, Green, White, or UV

3.3

2

25 mA

96 Ω

100 Ω

I copy pasted the table from this article (fancy name of copy paste with variation is reference) :

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http://www.evilmadscientist.com/2012/resistors-for-leds/

We roughly say to add lowest value resistor and when needed add a pull down resistor because we do not know what a newbie will do reading somewhat complicated circuit.

You actually need to calculate lesser as these days there are apps for Android, iOS and web which calculates the value of resistor for LED, like this :

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http://www.ohmslawcalculator.com/led-resistor-calculator

http://led.linear1.org/1led.wiz

What will happen for the multi color LED with four legs? This is example solution :